Much effort has been focused on developing means for processing audio signals to advance the listening experience. For example, electronic mixers are used to mix or blend together separate musical tracks or sequences to create a single performance. Other mechanisms have focused on enhancing the characteristics of a single musical track, e.g., by boosting certain desired frequencies and attenuating others through electronic equalization.
Some efforts have been made to adjust the tuning of electronic instruments. Proper tuning can mean the difference between a full, rich, resonating sound and a flat, lack-luster sound. Yet, due to physical constraints on instruments and the varying relationships of musical chords, proper tuning is difficult, if not impossible to attain throughout a musical performance. This problem is described in more detail below, with further background on the nature of musical chords.
In particular, there have been some efforts at what may be referred to as “variable tuning,” which makes pitch adjustments based on what notes are being played. For example, one type of variable tuning examines the chord being played by one or more instruments and makes pitch adjustments based on the chord. Some efforts at variable tuning have been made based on MIDI data, which some instruments are capable of generating. MIDI is a form of music notation that describes what note should be played and when. A MIDI file is a data stream of note events in a digital format that includes meta-information about each note to be played. This meta-information includes note characteristics such as pitch, attack, envelope, etc. MIDI note events can be generated by an electronic keyboard, for example. To hear the music represented by a MIDI file, the MIDI information is used to drive a tone generator or synthesizer at the specified pitch, with the specified attack and envelope characteristics.
Audio signals from other musical instruments (or vocals) have a substantially different format than MIDI data. For example, a musical instrument (or voice) provides signals that are transformed by a transducer (e.g., a microphone or guitar pick-up) for amplification, transmission and/or recording purposes. These signals typically describe how the amplitude of the input audio signal changes over time. For example, a typical audio signal in digital format is based on samples of the magnitude of the input audio signal, with the sampling rate being at least twice (typically greater) the highest frequency of interest.
As previously discussed, variable tuning may be based on what chord is being played. As used herein, the term “chord” means any combination of two or more notes played simultaneously. The ratio between the pitches of notes in a chord, affects the sound. Chords typically sound best when the instrument is tuned with what is referred to as “harmonic tuning”. With harmonic tuning the ratio of the pitch between notes in a scale can be represented by integers that are relatively small. For example, using harmonic tuning, the ratio of the pitch between the lowest note in a given scale and three notes higher (a small third) is precisely 6/5. As another example, the ratio of the pitch between the lowest note in a given scale and four notes higher (a large third) is precisely 5/4. As still another example, the ratio between the lowest note and seven notes higher (a pure fifth) is 3/2.
While harmonic tuning produces very good sounding chords, an instrument that is harmonically tuned can only play in a single key. The reason lies in the fact that the difference in pitch between successive notes in a scale is not uniform when using harmonic tuning. Hundreds of years ago, instruments such as harpsichords were tuned specifically for a single key (e.g., C-major, G-major, etc.). A collection of several harpsichords was therefore required to play different songs having different keys. Moreover, key changes within a song were impractical with harmonic tuning, as that would require a change of instruments.
Equal tempered tuning was invented to equalize the ratio of the pitch between successive notes in a scale. That is, if equal tempered tuning is being used, the pitch of each successive note in a scale is exactly two raised to the one-twelfth power times the pitch of the preceding note. Thus, the ratio of the pitch between A and A-sharp is precisely the same as the ratio between D and D-sharp, or between G-flat and G. Although the equal tempered tuning provides convenience and standardization across the scale, it does so at the expense of pure-sounding chords.
As previously discussed, some efforts have been made at variable tuning. One such effort is described in U.S. Pat. No. 5,442,129, issued to Werner Mohrlok. The Mohrlok patent discloses a chord recognition circuit, which ascertains at each MIDI input signal pattern (i.e., set of concurrent note events) corresponding to a chord, whether the input signal pattern corresponds to a chord pattern from a predetermined set of chord patterns. The chord patterns are stored in a chord table that has an entry for each chord pattern. Each entry has 12 attributes (corresponding to notes such as “A” “A-sharp”, etc), with each attribute describing whether that note is present or absent in the chord. When a chord recognition circuit ascertains that a MIDI input signal pattern corresponds to one of the predetermined chord patterns, a control circuit causes a signal pattern store circuit to emit an “optimally”tuned output signal pattern.
An underlying assumption in the Mohrlok patent is that the input data is based on the same tuning upon which the chord table is based. For example, if the input data is based upon equal tempered tuning, the assumption is that the chord table is also based upon equal tempered tuning. However, if the input data (e.g., MIDI data) does not conform to the same tuning as the chord table, then searching the chord table for a pattern (e.g., chords) that matches the pattern in the input signal will, at best, produce unpredictable results. For typical MIDI data the assumption that the input data is based on the same tuning upon which the chord table is based may be valid. However, for a more general class of input audio data, this assumption may not hold. For example, the input data may conform to a different type of tuning or may not conform to any standard or known form of tuning.
In view of the foregoing, there is a need for a mechanism to provide variable tuning for musical chords embodied in audio signals.
The approaches described in this section are approaches that could be pursued, but not necessarily approaches that have been previously conceived or pursued. Therefore, unless otherwise indicated, it should not be assumed that any of the approaches described in this section qualify as prior art merely by virtue of their inclusion in this section.